1. Field of the Invention
The present invention relates to devices for dampening noise, and particularly to multiple Helmholtz resonators that are acoustically coupled together to quickly and adjustably filter out more than one acoustic frequency.
2. Description of the Related Art
The Helmholtz resonator was first designed by Hermann von Helmholtz in the 1850s. The Helmholtz resonator has a cavity communicating with a main duct through a neck and is used to effectively attenuate narrow-band, low frequency noise. Narrow-band noise in the form of tonal noise is quite common in the case of rotating machinery, and in particular, in applications involving engine breathing systems. For example, an engine exhaust flow path may pass by an opening or throat of a Helmholtz resonator. Beyond the opening is a cavity in the Helmholtz resonator. The dimensions of the throat and cavity, in conjunction with the makeup of the gases involved, will determine the precise resonant frequency absorbed by the Helmholtz resonator.
The Helmholtz resonator is often looked at as an acoustic wave equivalent of a spring-mass system, where the spring represents the cavity and the mass represents the neck. Thus, the resonator's frequency and the transmission loss can be readily determined.
While Helmholtz resonators have been used to dampen specific frequencies, and multiple Helmholtz resonators can dampen a corresponding number of frequencies, it is often impractical to employ multiple, separate Helmholtz resonators. Even where the use of multiple Helmholtz resonators is not a problem, their use is ineffective in situations where the ideal frequencies to be filtered are not sufficiently static, especially where those frequencies change quickly. Thus, multiple Helmholtz resonators solving the aforementioned problems are desired.